A. Field of the Invention
The present invention relates generally to the field of genetic engineering and gene expression. More specifically, it relates to artificial tissues, particularly to artificial tissues comprising engineered cells having the ability to secrete insulin in a glucose-sensitive fashion. It also relates to the use of engineered cells in the production of human insulin for use in, for example, the treatment of insulin deficient conditions such as type I diabetes mellitus.
B. Description of the Related Art
The .beta. cells of the pancreatic islets of Langerhans synthesize insulin. A portion of this insulin is slowly and continually released into the bloodstream (basal secretion). However, the bulk of the insulin is stored in secretory vesicles and released only in response to certain physiological stimuli (stimulated secretion).
In humans and other mammals, the principal physiological stimulus for the secretion of insulin is increased blood levels of glucose (e.g., following ingestion of a carbohydrate meal). The capacity of normal islet .beta. cells to sense a rise in blood glucose concentration and to respond by secreting insulin is critical to control of blood glucose levels.
In certain insulin-deficient conditions such as diabetes mellitus, the pancreas loses its ability to manufacture and secrete insulin in response to rising blood glucose concentrations. The result is a metabolic imbalance, which causes blindness, kidney-related diseases, neurological disorders, cardiovascular diseases, non-accidental amputation of limbs, and death.
The current preferred treatment for insulin deficiency is injection of insulin once or twice daily. The objective of this regimen is to maintain glucose levels close to normal. However, daily insulin injections can not reproduce the rapid insulin secretory responses of normal islets to physiological demand. Most insulin deficient patients never achieve the finely-tuned glucose homeostasis needed to avoid long-term complications.
Consequently, a major goal of research into insulin deficiencies is to develop a system for sensing changes in blood glucose and quickly adjusting insulin output to maintain normal blood glucose levels at all times. One approach has been replacement of the malfunctioning organ by transplantation of normal pancreatic tissue. Lacy et al. (1986), Ann. Rev. Med., 37: 33-40; Lacey (July, 1995), Scientif. Amer. 273: 50-55. However, transplanted islets are recognized and destroyed by the same autoimmune mechanism responsible for destruction of the patient's original .beta. cells. For this reason artificial pancreatic devices containing live islets have often been designed to avoid immune rejection, often by enclosing islets in a semipermeable pouch or matrix which separates the transplanted islets from immunoreactive cells and molecules.
The treatment of diabetes with peritoneal implants of encapsulated islets in vivo diabetic models has been reported by several research groups. Lum et al. (1991), "Prolonged reversal of diabetic state in NOD Mice by xenografts of microencapsulated rat islets," Diabetes, 40: 1511; and Maki et al. (1991), "Successful treatment of diabetes with the biohybrid artificial pancreas in dogs," Transplantation, 51: 43; Scharp etal. (1990), "Insulin independence after islet transplantation into type I diabetic patient," Diabetes, 39: 515; Robertson (1991), "Pancreas Transplantation in humans with diabetes mellitus," Diabetes, 40: 1085; Colton et al. (1991), "Bioengineering in development of the hybrid artificial pancreas," J. Biomech. Eng., 113: 152; Reach (1990), "Bioartificial pancreas: Status and bottlenecks," Intern. J. Art. Organs, 13: 329; and Warnock et al. (1988), "Critical mass of purified islets that induce normoglycemia after implantation into dogs," Diabetes, 37: 467. See also, T. Matsumura (U.S. Pat. No. 3,827,565); Sun et al. (U.S. Pat. No. 4,323,457 (1982)); Chick etal. (U.S. Pat. Nos. 4,242,459 and 4,242,460 (1980)); Lim, U.S. Pat. No. 4,391,909; Loeb, U.S. Pat. No. 4,378,016; Newgard, U.S. Pat. No. 5,427,940 (1992); Bae, et. al., U.S. Pat. No. 5,262,055 (1993).